Thermal airflow sensor

ABSTRACT

A thermal airflow sensor includes a semiconductor device, a protective film a bonding wire, and a resin. The resin covers over a part of the semiconductor device so that the bonding wire is covered with the resin and the region including a thin-wall portion is exposed. The protective film is not covered with the resin and has an outer peripheral edge located outside the thin-wall portion.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/495,268, filed Apr. 24, 2017 which is a continuation of U.S.application Ser. No. 14/410,713, filed Dec. 23, 2014, which is a 371 ofInternational Application No. PCT/JP2013/065912, filed Jun. 10, 2013,which claims priority from Japanese Patent Application No. 2012-146286,filed Jun. 29, 2012, the disclosures of which are expressly incorporatedby reference herein.

TECHNICAL FIELD

The present invention relates to a sensor that detects physicalquantities. More particularly, the invention relates to a thermalairflow sensor.

BACKGROUND ART

Thermal airflow sensors have conventionally been a mainstream airflowsensor that is installed in the intake air passage of internalcombustion engines, such as those of automobiles, to measure intake airvolume since the thermal airflow sensors are capable of directlydetecting amount of air.

Recently, there has been developed an airflow sensor formed by havingresistors and insulating layer films deposited on a silicon substrate byuse of semiconductor micromachining technology, part of the siliconsubstrate being removed thereafter by a solvent represented by KOH toform a thin-wall portion. This airflow sensor is drawing attentionbecause it has high-speed responsiveness and is capable of detectingcounter flows thanks to its quick response. In recent years, for thepurpose of reducing the number of components constituting the substrateportion (printed substrate, silicon substrate, etc.), study has beenunderway to form a structure in which this airflow sensor is mounted ona lead frame of which the periphery is molded in resin.

PRIOR ART LITERATURE Patent Document

-   Patent Document 1: Japanese Patent No. 3610484

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

An existing thermal airflow sensor described in Patent Document 1 is aninvention in which the surface of a flow sensor element is provided witha protective film made of an organic material for the purpose ofimproving the reliability of the thin-wall portion formed by removingpart of the silicon substrate from its back side. According to PatentDocument 1, the insulating film of the thin-wall portion is givenenhanced resistance to dust. With this invention, however, there is roomfor consideration in partially exposing an area that includes thethin-wall portion in a structure in which the flow sensor element isadhesively attached to a member such as the lead frame, with theperiphery of the flow sensor element sealed with resin.

When resin molding is performed for partial exposure, it is generalpractice to press a metal mold, an insertion die or the like onto theperiphery of the thin-wall portion over a semiconductor detectionelement during molding so that a mold resin material will not be formedin the exposed portion. A principal method for pressing the insertiondie involves controlling the amount of movement of the insertion die.Where mass production is considered, the amount of movement to be set isalways constant; the amount of movement is left unadjusted from oneproduct to another. At this point, if the pressing force on theinsertion die is not sufficient, the mold resin could flow into theexposed portion. To avoid this eventuality requires pressing theinsertion die toward the semiconductor device with a certain level offorce. If the pressing force is excessive, the semiconductor device canbe deformed. Thus where the area including the thin-wall portion is tobe partially exposed when sealed with resin, the force with which topress the insertion die has a margin to certain extent.

Also, there are variations in film thickness as well as in adhesivethickness over the detection element from one product to another. As aresult, the height of the semiconductor device mounted on the lead framevaries. It follows that that the force applied from the insertion die orthe contact distance thereto varies in each product. This furtherreduces the permissible range of the pressing force toward the insertiondie, leading to a decline in throughput yield.

An object of the present invention is to improve the reliability of aproduct in which a semiconductor device is partially exposed when sealedwith resin.

Means for Solving the Problem

In achieving the above object and according to the present invention,there is provided a thermal airflow sensor including: a semiconductorsubstrate having a thin-wall portion, a heating resistor provided overthe thin-wall portion, and resistance temperature detectors installedupstream and downstream of the heating resistor; a protective filmprovided over the semiconductor substrate; and a resin that seals thesemiconductor substrate, the resin further including an exposure portionfor partially exposing an area including the thin-wall portion. Theprotective film is provided in a manner seamlessly enclosing the heatingresistor, the protective film having an outer peripheral edge locatedoutside the thin-wall portion and over the exposure portion.

Effect of the Invention

The present invention improves the reliability of a product in which asemiconductor device is partially exposed when sealed with resin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a set of structural views showing a sensor element yet to bemolded as a first embodiment of the present invention, where FIG. 1(a)is a lateral sectional view and FIG. 1(b) is an overhead topographicview.

FIG. 2 is a set of structural views showing the sensor element molded asthe first embodiment, where FIG. 2(a) is a lateral sectional view andFIG. 2(b) is an overhead topographic view.

FIG. 3 is a schematic explanatory view showing how molding is performedon the first embodiment.

FIG. 4 is a schematic explanatory view showing how the mold resin flowsout in the first embodiment.

FIG. 5 is a set of structural views of a sensor element yet to be moldedas a second and a third embodiment of the present invention, where FIG.5(a) is a lateral sectional view and FIG. 5(b) is an overheadtopographic view.

FIG. 6 is a set of structural views of the sensor element molded as thesecond embodiment, where FIG. 6(a) is a lateral sectional view and FIG.6(b) is an overhead topographic view.

FIG. 7 is a schematic explanatory view showing how molding is performedon the second embodiment.

FIG. 8 is a schematic explanatory view showing how the mold resin flowsout in the second embodiment.

FIG. 9 is an explanatory view of a slit.

FIG. 10 is a set of structural views showing the sensor element moldedas the third embodiment, where FIG. 10(a) is a lateral sectional viewand FIG. 10(b) is an overhead topographic view.

FIG. 11 is a schematic explanatory view showing how molding is performedon the third embodiment.

FIG. 12 is a schematic explanatory view showing how the mold resin flowsout in the third embodiment.

FIG. 13 is a structural view showing a sensor element yet to be moldedas a fourth embodiment of the present invention.

FIG. 14 is a structural view showing a sensor element yet to be moldedas a fifth embodiment of the present invention.

FIG. 15 is a structural view showing a thermal airflow sensor accordingto the present invention.

MODE FOR CARRYING OUT THE INVENTION

The thermal airflow sensor according to the present invention will nowbe explained with reference to FIG. 15.

The thermal airflow sensor of the present invention includes a housing 3and a semiconductor package 2 installed inside an intake pipe 5 thatfeeds intake air 1 to an automobile internal combustion engine (notshown).

The housing 3 includes a connector terminal 8 coupled electrically tothe semiconductor package 2, a flange portion 4 that fixes the housing 3to the intake pipe 5, and an auxiliary passage 6 that admits part of theintake air 1.

The semiconductor package 2 is formed by having a lead frame 10, asemiconductor substrate 20, circuit elements, and a temperature sensorsealed integrally with a mold resin 60. The semiconductor package 2 alsohas a partially exposed area (not covered with the mold resin 60) so asto expose a flow rate detection portion 7 to the intake air. The flowrate detection portion 7 is installed inside the auxiliary passage 6 andcalculates the flow rate of the intake air 1 from the flow rate of afluid flowing through the auxiliary passage 6.

The first embodiment of the present invention will now be explained withreference to FIGS. 1 through 4.

The structural views of the sensor element constituting the firstembodiment of this invention will be explained below with reference toFIGS. 1 and 2. Here, FIG. 1 is a set of structural views of the sensorelement yet to be molded as the first embodiment, and FIG. 2 is a set ofstructural views of the sensor element molded as the first embodiment.

As shown in FIG. 1, the thermal airflow sensor has a semiconductorsubstrate 20 typically made of silicon and deposited with insulatingfilm and resistor layers. A thin-wall portion 25 is formed by removingpart of the back side of the semiconductor substrate 20 typically by useof potassium hydroxide (KOH). A heating resistor 21, an upstreamresistance temperature detector 22, and a downstream resistancetemperature detector 23 are formed over the thin-wall portion 25. Thetemperature of the heating resistor 21 is subjected to feedback controlso that the temperature of the heating resistor 21 remains higher thanthat of the intake air 1 by predetermined degrees. The flow rate of theintake air 1 is measured from information about the difference betweenthe temperature measured by the upstream resistance temperature detector22 and the temperature measured by the downstream resistance temperaturedetector 23. An organic protective film 30 typically made of polyimideis formed over the surface of the thermal airflow sensor. The organicprotective film 30 is coated once, uniformly, all over the sensorsurface by use of a coating machine such as a spinner. Thereafter, thecoat is partially removed by etching through patterning technology toform a stagger between the semiconductor substrate 20 and the organicprotective film 30. The organic protective film 30 is shaped seamlesslyto enclose the heating resistor 21. Because the flow rate of the intakeair is measured with the heating resistor 21, upstream resistancetemperature detector 22 and downstream resistance temperature detector23, these three detectors need to be exposed to the intake air, so thatthey are not covered with the organic protective film. Also, AI wiring40 is formed over the surface of the thermal airflow sensor. The AIwiring 40 is electrically coupled to the lead frame 10 by means of abonding wire 50 such as a gold wire. The semiconductor substrate 20 isfixed to the lead frame 10 with adhesive or the like.

As shown in FIG. 2, the semiconductor substrate 20 and lead frame 10 aresealed with the mold resin 60. Here, because the heating resistor 21,upstream resistance temperature detector 22 and downstream resistancetemperature detector 23 need to be exposed to a medium to be measuredfor flow rate detection, there is provided a structure in which an areaincluding the flow rate detection portion 7 is partially exposed fromthe mold resin 60 (not covered therewith). Furthermore, the peripheraledge of the organic protective film 30 enclosing the heating resistor 21is located outside the thin-wall portion 25 in a manner partiallyexposing the organic protective film 30. With this structure, even ifthe resin leaks out of between the surface of the thermal airflow sensorand the insertion die 83 during molding, the leak is stemmed by theorganic protective film 30 so that the resin will not reach thethin-wall portion 20.

Molding on the first embodiment will be explained below with referenceto FIGS. 3 and 4. FIG. 3 is a schematic explanatory view showing howmolding is performed on the first embodiment, and FIG. 4 is a schematicexplanatory view showing how the mold resin flows out in the firstembodiment.

As shown in FIG. 3, a semiconductor package of a partially exposedstructure is formed with a lower metal mold 80, an upper metal mold 81,and the insertion die 83 provided to be inserted into the upper metaldie 81. The thermal airflow sensor having the semiconductor substrate 20mounted on the lead frame 10 is sandwiched between the lower metal mold80 and the upper metal mold 81. The insertion die 83 is pressed towardthe thin-wall portion 25 and other parts to be partially exposed so thatthese locations will not be covered with the mold resin 60. The resin isthen allowed to flow in through an insertion opening 82 to produce thesemiconductor package of the partially exposed structure. The insertionopening 82 may be provided through either the lower metal mold 80 or theupper metal mold 81. When the pressing portion of the insertion die 83is pressed toward the substrate surface, the area toward which theinsertion die 83 is pressed is not sealed with the resin and thusexposed partially. However, since the thin-wall portion 25 is thinnerthan the other portions, pressing the insertion die 83 directly towardthe thin-wall portion 25 can deform the thin-wall portion 25, resultingin flow rate detection errors. Thus the insertion die 81 is structuredto have a concave part on its pressing portion so that the thin-wallportion 25 will be fitted into the concave part. The pressing portionformed on the peripheral edge of the concave part is pressed toward thesubstrate surface so that the insertion die 83 will not come into directcontact with the thin-wall portion 25 upon sealing with the resin. Thisprevents the load with which to press the insertion die 83 from beingapplied to the thin-wall portion 25, so that the thin-wall portion isprevented from getting deformed when the area including the thin-wallportion is partially exposed during sealing with the resin.

As shown in FIG. 4, if the load with which to press the insertion die 83is insufficient, a gap may be formed between the surface of the thermalairflow sensor and the insertion die. If the resin is poured in thisstate, the resin may flow out through the gap 61 between the insertiondie 83 and the surface of the thermal airflow sensor. However, the firstembodiment of the present invention offers a structure in which theorganic protective film 30 encloses the heating resistor 21 and islocated in an area partially exposed from the mold resin 60. In thisstructure, the organic protective film 30 stems the resin 60 that mayleak out through the gap 61, so that the leaking resin is controlledfrom reaching the thin-wall portion 25. Even if the force with which topress the insertion die 83 is insufficient and even if a product ismanufactured in which the mold resin 60 leaks to the semiconductordevice portion, the performance specification of the product are met aslong as the leak does not reach the thin-wall portion 25 that is thesensing area. Thus according to the first embodiment of the presentinvention, the reliability of the thermal airflow sensor is ensured evenwhere insufficient load on the insertion die 83 results in resinleakage.

Here, consider the case where the insertion die 83 is pressed undermovement control. Since there are variations in the height of thesurface of the semiconductor substrate 20 from one product to another, asemiconductor substrate 20 with a higher height than usual is subject togreater load than usual. Too much load can deform the sensor element. Onthe other hand, a semiconductor substrate with a lower height than usualforms the gap 61 between the insertion die 83 and the surface of thethermal airflow sensor, and the resin may leak through the gap 61.According to the first embodiment of the present invention, thereliability of the thermal airflow sensor is ensured even when the loadon the insertion die 83 is insufficient. This means that during massproduction, the manufacturing margin may be increased in a mannerfavoring lower load on the insertion die 83. That in turn improvesthroughput yield.

Moreover, since the thin-wall portion 25 is made of an inorganicmaterial and formed thin and fragile in order to boost thermalinsulation characteristics, the thin-wall portion 25 needs to have itsstrength improved against the impact of dust. In particular, theperipheral edge of the thin-wall portion 25 is more vulnerable to theimpact of dust than the other portions. Thus an inner peripheral edge ofthe organic protective film 30 is positioned at the thin-wall portion 25as shown in FIG. 1 so that the peripheral edge of the thin-wall portion25 is covered with the organic protective film 20 that absorbs the shockof the impact of dust. With this structure, the strength of thethin-wall portion 25 against the impact of the dust included in theintake air is raised. The thermal airflow sensor consequently improvesits contamination resistance and higher reliability in itsimplementation.

The second embodiment of the present invention will now be explainedwith reference to FIGS. 5 through 9. The same structures as those of thefirst embodiment will not be discussed further.

The structural views of the sensor element constituting the secondembodiment of this invention will be explained below with reference toFIGS. 5 and 6. Here, FIG. 5 is a set of structural views of the sensorelement yet to be molded as the second embodiment, and FIG. 6 is a setof structural views of the sensor element molded as the secondembodiment.

As shown in FIG. 5, the organic protective film 30 provided over thesemiconductor substrate 20 has an exposure portion for partiallyexposing the thin-wall portion 25 so that the heating element 21,upstream resistance temperature detector 22, and downstream resistancetemperature detector 23 are exposed to the medium to be measured, theorganic protective film 30 also having a slit 35 formed in a mannerenclosing the thin-wall portion 25. The slit 35 seamlessly encloses thethin-wall portion 25 so that even if resin leakage occurs during resinmolding, the slit 35 traps the mold resin 60 and prevents it fromflowing to the thin-wall portion 25. The organic protective film 30 isprovided to protect the rim of the thin-wall portion 25, which booststhe strength of the thin-wall portion 25 against the impact of the dustincluded in the intake air. The slit 35 partially exposes thesemiconductor substrate 20 from the organic protective film 30, thusforming a stagger between the exposed surface of the semiconductorsubstrate 20 and the organic protective film 30. Preferably, thereshould be provided a structure in which the AI wiring 40 is covered withthe organic protective film 30 for protection against corrosivecomponents such as water.

And as shown in FIG. 6, the semiconductor substrate 20 and lead frame 10are sealed with the mold resin 60 in a manner partially exposing theentire inner peripheral edge and the outer peripheral edge of the slit35 from the mold resin 60. With the entire inner peripheral edge of theslit 35 located in an area partially exposed from the mold resin, evenif the mold resin 60 leaks out through the gap 61 as shown in FIG. 8,the slit 35 traps the mold resin 60 and prevents it from reaching thethin-wall portion 25.

Molding on the second embodiment will be explained below with referenceto FIGS. 7 and 8. FIG. 7 is a schematic explanatory view showing howmolding is performed on the second embodiment, and FIG. 8 is a schematicexplanatory view showing how the mold resin flows out in the secondembodiment.

As shown in FIG. 7, when the semiconductor package of a partiallyexposed structure of the second embodiment is produced, the pressingportion of the insertion die 83 is pressed toward the organic protectivefilm 30. Because the organic protective film 30 acts as a buffermaterial that absorbs the stress propagated to the thin-wall portion 25,deformation of the thin-wall portion 25 is restrained during molding.Thus according to the second embodiment of the present invention,detection errors attributable to the deformation of the thin-wallportion 25 are restrained, and hence that the reliability of the thermalairflow sensor is improved.

Further advantages of providing the slit 35 in the organic protectivefilm 30 will be explained below with reference to FIG. 9.

In a structure where the mold resin 60 is applied to the thermal airflowsensor with the organic protective film 30 interposed therebetween, theorganic protective film 30 is stressed due to resin contraction aftermolding. Where the organic protective film 30 is shaped to communicatewith the thin-wall portion edge, the stress caused by resin contractionof the mold resin 60 may reach the edge of the thin-wall portion 25 andaffect flow rate characteristics. According to the second embodiment ofthis invention, however, the slit portion 35 is formed in a mannerisolating an organic protective film 31 from an organic protective film32 formed over the thin-wall portion edge, the organic protective film31 being positioned in an area where the mold resin 60 is in contactwith the thermal airflow sensor. With this structure, the stress doesnot reach the organic protective film 32 formed over the thin-wallportion edge by way of the organic protective film 30. This provides anadvantage of reducing the stress-induced effects on flow ratecharacteristics.

The third embodiment of the present invention will now be explained withreference to FIGS. 10 through 12. The same structures as those of thesecond embodiment will not be discussed further.

As shown in FIGS. 10 and 11, during resin molding, a slit innerperiphery-side organic protective film 33 is positioned in a mannerpartially exposed from the mold resin 60, and a slit outerperiphery-side organic protective film 34 is covered with the mold resin60. With the slit inner periphery-side organic protective film 33located in an area partially exposed from the mold resin 60, even if themold resin 60 flows out through the gap 61 as shown in FIG. 12, the moldresin 60 is stemmed by the protective film 33 and thereby prevented fromreaching the thin-wall portion 25.

Whereas the organic protective film 30 protects the AI wiring 40 fromcorrosive components such as water, there is fear that the organicprotective film 30 itself may absorb water and transfer it to the AIwiring 40. In the structure according to the third embodiment of thisinvention, the organic protective film 34 covering the AI wiring 40 inthe mold resin 60 does not come into direct contact with air. Thestructure thus prevents corrosion of the AI wiring. Furthermore, theorganic protective film 34 is capable of stemming corrosive componentssuch as water coming in through the interface between the semiconductorsubstrate 20 and the mold resin 60, thereby reducing the infiltration ofcorrosive components including water into the AI wiring 40. Thus in thestructure according to the third embodiment of this invention, possiblecorrosion of the AI wiring 40 is further reduced and reliability isimproved accordingly.

Moreover, as with the second embodiment, the protective film sandwichedby the mold resin and the semiconductor substrate is independent of theprotective film formed over the thin-wall portion. This structure helpslower the stress-induced effects on the thin-wall portion.

The fourth embodiment of this invention will now be explained withreference to FIG. 13.

Whereas each of the slits of the first embodiment are placed distantlyover the entire periphery, the effect of preventing the leakage of themold resin is still obtained even when the slit is formed on one sidealone or in one direction only.

If it is known beforehand that the insertion die tends to be in unevencontact with the semiconductor substrate 20, the direction in which agap is highly likely to occur can be identified. If the slit is formedin that direction, the slit prevents the leaking mold resin 60 fromreaching the thin-wall portion 25, such that the throughput yield can besignificantly improved.

The same applies to the above-mentioned stress-induced effects. If thestress of the resin is expected to occur in a specific direction throughevaluation of actual products and/or through analysis, the slit may beformed in that direction so as to improve the reliability of thethin-wall portion effectively.

The fifth embodiment of the present invention will now be explained withreference to FIG. 14.

Whereas the slit of the second embodiment is shaped as nestedcircumferences with space interposed, staggered multiple slits formed asshown in FIG. 14 also provide the effect of preventing theabove-mentioned leakage of the mold resin.

One object of forming multi-staggered slits is to protect a resistor 37,formed over the semiconductor substrate, from the impact of dust. Whereit is desired, as in the case of the thin-wall portion 25, to expose atemperature sensor 37 formed over the semiconductor substrate for thesake of better thermal responsiveness, the protective film 31 needs tobe formed inside the slit provided in the second embodiment. In thiscase, multiple slits are formed staggered to prevent leakage of the moldresin 60. In such a structure, the multi-staggered slits are alsoeffective in preventing the mold resin 60 from leaking.

In the first through the fifth embodiments, the organic protective film30 should preferably be made of polyimide. While the thin-wall portion25 is subject to high temperatures as a result of the heating resistor21 being heated so as to measure the flow rate of intake air, polyimidehas good resistance to heat and minimizes heat-induced degradation ofthe material. This makes it possible to improve the strength of ameasuring element 1 against the impact of solid particles for anextended period of time.

In a structure where the mold resin 60 is applied to the thermal airflowsensor with the organic protective film 30 interposed therebetween, theorganic protective film 30 is stressed due to resin contraction aftermolding. Where the organic protective film 30 is shaped to communicatewith the thin-wall portion edge, the stress caused by resin contractionof the mold resin 60 may reach the edge of the thin-wall portion 25 andaffect flow rate characteristics. According to the second embodiment ofthis invention, however, the slit portion 35 is formed in a mannerisolating an organic protective film 30 from an organic protective film32 formed over the thin-wall portion edge, the organic protective film30 being positioned in an area where the mold resin 60 is in contactwith the thermal airflow sensor. With this structure, the stress doesnot reach the organic protective film 32 formed over the thin-wallportion edge by way of the organic protective film 30. This provides anadvantage of reducing the stress-induced effects on flow ratecharacteristics.

When the organic protective film 30 is made of polyimide, there can beprovided a thermal airflow sensor that improves its strength of theinsulating film over the thin-wall portion toward dust and yet controlsthe drop in throughput yield without increase in cost, even with thesemiconductor device sealed with the resin in a manner being partiallyexposed.

REFERENCE NUMERALS

-   1 Intake air-   2 Semiconductor package-   3 Housing-   4 Flange-   5 Intake pipe-   6 Auxiliary passage-   7 Flow rate detection portion-   8 Connector terminal-   10 Lead frame (substrate support member)-   20 Semiconductor substrate-   21 Heating resistor-   22 Upstream resistance temperature detector-   23 Downstream resistance temperature detector-   25 Thin-wall portion-   30 Organic protective film-   31 Organic protective film-   33 Slit inner periphery-side organic protective film-   34 Slit outer periphery-side organic protective film-   35 Slit-   36 Slit-   37 Resistor formed over semiconductor substrate-   38 Temperature sensor formed over semiconductor substrate-   40 AI wiring-   50 Bonding wire-   60 Mold resin-   61 Boundary between mold resin and thermal flow sensor-   80 Lower metal mold-   81 Upper metal mold-   82 Resin pouring hole-   83 Insertion die

The invention claim is:
 1. A thermal airflow sensor comprising: asemiconductor device having a thin-wall portion on which a heatingresistor and resistance temperature detectors are provided; a protectivefilm formed on a surface of the semiconductor substrate so that theheating resistor is exposed; a bonding wire connected to thesemiconductor device electrically; a resin that covers over a part ofthe semiconductor device so that the bonding wire is covered with theresin and the region including the thin-wall portion is exposed; whereinthe protective film has multiple slits between the region covered withthe resin and the thin-wall portion.
 2. The thermal airflow sensoraccording to claim 1, further comprising: a resistor formed oversemiconductor substrate; wherein the slits are formed so as to exposethe resistor.